1. Technical Field
One or more embodiments of the present invention relate to an acoustic sensing element (MEMS micro chip) that is prepared by utilizing a MEMS technology and an acoustic sensor provided with the acoustic sensing element.
2. Related Art
FIG. 1A is a sectional view illustrating a structure of a conventional acoustic sensor, FIG. 1B is a schematic plan view illustrating a state in which a cover of the acoustic sensor is removed, and FIG. 1C is a plan view illustrating a state in which the cover of the acoustic sensor is attached. In an acoustic sensor 11, an acoustic sensing element 14 and a processing circuit unit 15 (IC chip) are accommodated in a casing including a base substrate 12 and a cover 13, a lower surface of the acoustic sensing element 14 is bonded to an upper surface of the base substrate 12 by a thermosetting bonding agent 24, and the processing circuit unit 15 is fixed by a bonding agent. The acoustic sensing element 14 includes a silicon substrate 16 in which a vertically-piercing back chamber 17 is formed, and a thin-film vibration electrode plate 18 is disposed opposite an upper surface opening of the back chamber 17 in an upper surface of the silicon substrate 16 and is covered with a fixed electrode plate 19 that is provided opposite the vibration electrode plate 18. In the vibration electrode plate 18, four corners are supported by leg portions 20, and a portion except the four corners is supported in the air by the upper surface of the silicon substrate 16. An opening 21 is formed in the cover 13 in order to guide an acoustic vibration into the casing, and a plurality of acoustic holes 22 are made in the fixed electrode plate 19 in order to guide the acoustic vibration to the vibration electrode plate 18. The acoustic vibration is converted into an electric signal and outputted based on an electrostatic capacitance change between the vibration electrode plate 18 and the fixed electrode plate 19. The vibration electrode plate 18 vibrates by resonating with the acoustic vibration, thereby generating the electrostatic capacitance change (for example, see International Patent Publication No. 2008-510427).
(Run-Up of Bonding Agent)
However, in the acoustic sensor 11, because the lower surface of the silicon substrate 16 having the vertically-piercing back chamber 17 is bonded to the upper surface of the base substrate 12 by the thermosetting bonding agent 24, the bonding agent 24 that is in the fluidized state immediately after application runs up the wall surface of the back chamber 17 by a surface tension to easily reach the upper surface of the silicon substrate 16. Particularly, in the case of the back chamber 17 having a prismatic shape or a truncated-pyramid shape, the bonding agent 24 easily runs up to the upper surface of the silicon substrate 16 along a valley line (that is, an angular portion between sidewall surfaces of the back chamber 17). When the bonding agent 24 runs up to the upper surface of the silicon substrate 16, the bonding agent 24 invades into a gap (vent hole 23) between the upper surface of the silicon substrate 16 and the lower surface in an edge portion of the vibration electrode plate 18, and a part of the vibration electrode plate 18, which should originally be floated from the silicon substrate 16, is stuck to the silicon substrate 16 to suppress the vibration of the vibration electrode plate 18. When the vibration electrode plate 18 and the silicon substrate 16 are stuck by the run-up of the bonding agent 24, a distance between the fixed electrode plate 19 and the vibration electrode plate 18 is widened to decrease the electrostatic capacitance value, which causes sensitivity degradation of the acoustic sensor 11 or a characteristic change. When the run-up of the bonding agent 24 is prevented, a range of options of the bonding agent 24 is narrowed and cost is increased.
(Silicon Substrate Deformation Due to Heat and External Force)
In the acoustic sensor 11 having the above-described structure, according to one or more embodiments of the present invention, the base substrate 12 is made of a hard material such as ceramic whose linear expansion coefficient is substantially equal to that of the silicon substrate 16. However, the ceramic substrate is expensive, and the cost of the acoustic sensor 11 is increased. Therefore, an organic substrate such as a resin substrate and a resin multi-layer substrate is generally used.
Because the organic substrate significantly differs from the silicon substrate 16 of the acoustic sensing element 14 in the linear expansion coefficient, a thermal stress is generated between the silicon substrate 16 and the base substrate 12 by heat generation of the processing circuit unit 15 or external heat, which generates a risk of a warpage in the silicon substrate 16 and the base substrate 12. Therefore, a soft (low Young's modulus) bonding agent is used as the bonding agent 24 that bonds the lower surface of the silicon substrate 16 to the base substrate 12, and an internal stress generated between the base substrate 12 and the silicon substrate 16 is relaxed by the soft bonding agent 24.
The bonded portion between the silicon substrate 16 and the base substrate 12 has the flexible structure, and the silicon substrate 16 also has relatively low rigidity because the silicon substrate 16 is vertically pierced by the back chamber 17. Therefore, the silicon substrate 16 is easily deformed when the thermal stress is generated or when the external force is applied.
When the silicon substrate 16 is deformed, the distance between the vibration electrode plate 18 and the fixed electrode plate 19 is decreased to degrade the sensitivity or the characteristic is changed, which results in a possibly that the acoustic sensor 11 cannot be used. Therefore, the gap distance between the vibration electrode plate 18 and the fixed electrode plate 19 cannot extremely be decreased, which causes a restriction in improving the sensitivity of the acoustic sensor 11.
(Brittleness of Vibration Electrode Plate)
When the silicon substrate 16 is deformed, a stress is unevenly generated in the vibration electrode plate 18 to decrease strength of the vibration electrode plate 18. Additionally, in order to sense the weak acoustic vibration with the acoustic sensor, it is necessary to extremely thin the vibration electrode plate 18. Therefore, the vibration electrode plate 18 is considerably brittle, and the vibration electrode plate 18 is easily broken by a drop of the acoustic sensor. Accordingly, in consideration of the strength, the thickness of the vibration electrode plate 18 is hardly decreased than ever before.
(Back Chamber Leakage Due to Bonding Failure)
When the silicon substrate 16 or the base substrate 12 is deformed, possibly the bonding agent 24 is peeled off to generate a bonding failure. Occasionally a gap is generated between the lower surface of the silicon substrate 16 and the base substrate 12 by the bonding failure when the lower surface of the silicon substrate 16 is bonded to the base substrate 12 by the bonding agent 24. When the bonding failure is generated, the acoustic vibration invades from the gap to reduce a difference in sound pressure between a surface and a rear surface of the vibration electrode plate 18, which degrades the sensitivity of the acoustic sensor 11.